Digital halftoning of images is a process well known in the art and is normally used in display or printing devices having only bi-level capabilities of display or printing. As a result, halftoning techniques are often used in the binary representation of grey scale images in a wide range of applications such as laser printers, facsimile machines, lithography (newspaper printing), liquid crystal displays, plasma panels and other forms of flat panel displays. Gray scale images are typically converted to binary images using various halftoning techniques. Color images are normally divided into their various primary colors, for example, Red, Green and Blue (RGB) primaries are commonly used in video display devices, and Cyan, Magenta, Yellow and Black (CYMK) primaries are commonly used in printing devices, and the various halftoning techniques can be applied independently to each primary color. For a thorough discussion of the principles of halftoning, reference is made to a standard textbook, such as Digital Halftoning by R. Ulichney, MIT Press, Cambridge, Mass. (1990).
One particular halftoning technique is known as dithering. This technique involves a point-wise comparison of portions of the input image with a non-image, usually being an aperiodic array or mask. For every point or pixel in the image, depending on which of the point values of the image or the mask is larger, either an on or off value is used at the corresponding location in the binary output image. Again, with full color images the above process can be applied to each primary color channel independently.
While dithering has the advantage of simplicity and speed over other halftoning processes, dithering results in poor edge sharpness at edges of graphical objects forming part of (or the entire) image. Hence, it is desirable to have a halftoning process which has the favorable aspects of dithering such as speed and simplicity, but provides improved output image quality over point-process halftoning techniques such as dithering.
It is an object of the present invention to provide an alternative form of halftoning which leads to improved outputs, at least for a class of images. The class of images includes, for example, images where pixels are halftoned concurrently, images to be halftoned in a parallel blockwise manner or images to be halftoned in a sequential blockwise manner.